A photodiode has been widely employed as a photodetector that operates at high speed in an optical transmission device. Similarly, in an optical fiber radio communication device, the photodiode is essential as a key device. The optical fiber radio communication device is a transmission device having various advantages by which the transmission of a broad band and high capacity is enabled, various processor circuits can be downsized because the wavelength of light is short, a transmission loss is very low, and an electromagnetic interference is extremely reduced.
In the optical fiber radio communication device, light is modulated according to an RF electric signal and transmitted, and the modulated light is detected by a photodiode, to thereby reproduce an original RF electric signal. The reproduced RF electric signal is transmitted to a succeeding processor circuit or radiated toward a space by means of an antenna.
In general, an electric signal that is output from the photodiode is very weak. For that reason, when the electric signal that is extracted from the photodiode is transmitted to a circuit at a post-stage, it is necessary to amplify the signal with the aid of an amplifier circuit such as amplifier. For example, even in a case where the signal that has been transmitted through the optical fiber radio communication is radiated toward the space from the antenna, it is necessary to provide an electric power for radiating the electric signal toward the space after the electric signal has been detected by the photodiode, so the electric signal needs to be amplified by means of an amplifier circuit.
However, a power supply is required in an amplifier circuit such as an amplifier involving a power consumption. In addition, a space for the amplifier circuit is required, so it is difficult to downsize the device. Further, an electric power line that supplies an electric power is required, and an electromagnetic interference occurs due to an electromagnetic field from the electric power line, thereby making it impossible to sufficiently utilize the features of the optical fiber radio communication device with no electromagnetic interference.
As a method of increasing the electric power of an output electric signal from a photodiode, there has been proposed a traveling wave photodiode (for example, refer to Patent Document 1). This inputs a light to a plurality of photodiodes with a delay time bits by bits, and extracts output electric signals gradually with a delay time so as to cancel the delay time that has been given previously. However, in this method, since it is necessary to perfectly match those delay times with each other, and the impedance matching at the time of combining the electric powers is difficult, the output electric power that is actually obtained is not yet limited under the existing circumstances.
Patent Document 1: WO 2003-516641